Silicon carbide (below, referred to as “SiC”) is a wide bandgap semiconductor having a 2.2 to 3.3 eV wide bandgap. Due to its excellent physical and chemical properties, it has been the focus of R&D as an environmentally resistant semiconductor material. In particular, in recent years, it has been focused on it as a material for blue to ultraviolet short wavelength devices, high frequency electronic devices, high voltage resistant, high output electronic devices, etc. There has been a boom in R&D in the fabrication of devices by SiC.
For promoting practical application of SiC devices, production of large size SiC single crystals is essential. In most cases, the method of using the physical vapor transport method (Rayleigh method or improved Rayleigh method) to grow a bulk SiC single crystal has been employed (see NPLT 1). That is, an SiC material for sublimation is placed in a crucible, a seed crystal consisting of an SiC single crystal is attached to a lid of the crucible, and the material is made to sublimate to thereby grow an SiC single crystal on the seed crystal by recrystallization. Further, after a substantially columnar shaped SiC bulk single crystal (below, “SiC single-crystal ingot”) is obtained, in general the SiC single-crystal ingot is cut into a 300 to 600 μm or so thickness. By this, an SiC single-crystal wafer is produced. The SiC single-crystal wafer is used for fabrication of SiC devices in the electrical and electronic fields etc.
In this regard, an SiC single crystal contains not only hollow hole-shaped defects running in the growth direction called “micropipes”, but also dislocation defects, stacking faults, and other crystal defects. These crystal defects cause a drop in the device performance, so their reduction is an important factor in the application of SiC devices. Among these, dislocation defects include threading edge dislocations, basal plane dislocations, and threading screw dislocations. For example, there is a report that in the commercially available SiC single crystal wafers, there are 8×102 to 3×103 (/cm2) threading screw dislocations, 5×103 to 2×104 (/cm2) threading edge dislocations, and 2×103 to 2×104 (/cm2) basal plane dislocations (see NPLT 2).
In recent years, the research and investigation relating to SiC crystal defects and device performance have been advanced very much. The facts that threading screw dislocation defects become the cause of leakage of current in devices, and decrease the lifetime of gate oxide films, etc. have been reported (see NPLTs 3 and 4). To fabricate high performance SiC devices, an SiC single-crystal ingot reduced in threading screw dislocation density has been sought.
Here, there are examples of reports of the behavior of threading screw dislocations in the physical vapor transport method (see NPLT 5). That is, according to NPLT 5, at the interface of a seed crystal consisting of an SiC single crystal and an SiC single crystal grown thereon, at the side of the SiC single crystal grown on the seed crystal (below, referred to as the “grown SiC single crystal”), the threading screw dislocation density increases greatly once compared with the density at the seed crystal side, and then falls along with growth of the SiC single crystal. Because of this reason, as one method of obtaining an SiC single-crystal ingot reduced in threading screw dislocation density, the method for causing this behavior of dislocations at the interface as early as possible is expected to be effective.